Even in a novel that is heavy with death and grief and suffering, our characters find humor—not the slapstick sort, but the slightly inappropriate, wry variety of humor. It's not that the characters are irreverent about everything, but they are irreverent about things that make them uncomfortable, or things that are Debbie Downers and so they need to laugh to cope with them. Think about what the Colonel says on the way to Alaska's funeral:
"I don't suppose I can wear the flamingo tie," he said as he pulled on black socks.
"It's a bit festive, given the occasion," I responded.
"Can't wear it to the opera," said the Colonel, almost smiling. "Can't wear it to a funeral. Can't use it to hang myself. It's a bit useless, as ties go." I gave him a tie. (6after.3-5)
Or when Miles and the Colonel have to listen to their classmates (the ones who pranked Alaska) wax eloquently about a person they barely knew.
The Colonel and I said nothing, while a bunch of people who didn't know Alaska extolled her virtues and professed to be devastated, and at first, it bothered me. I didn't want the people she didn't know—and the people she didn't like—to be sad. They'd never cared about her, and now they were carrying on as if she were a sister. But I guess I didn't know her completely, either. […]
So they didn't bother me, really. But next to me, the Colonel breathed slowly and deeply through his nose like a bull about to charge.
He actually rolled his eyes. (8after.12-14)
Even in his grief, the Colonel gets annoyed about insincerity—which reminds us that though they're an irreverent bunch, they're also capable of being quite serious, of reflecting on the gravity of their lives and life in general.
Green understands the paradoxical truth that life events have both more and less meaning than they should for many teenagers. So when Miles gets concussed, it's hugely important (though not so much in terms of the book as a whole), but the revelation that Alaska's mother died in front of her becomes "just another worst day, albeit the worst of the bunch" (2before.80) (though we readers recognize that this is a key moment in both Alaska's life and in the novel).
Miles provides most of the reflection because he's our narrator. When Alaska tries to play of her mother's death as just another worst day, Miles knows the truth:
She was scared, sure. But more importantly, maybe she'd been scared of being paralyzed by fear again.
"We are all going," McKinley said to his wife, and we sure are. There's your labyrinth of suffering. We are all going. Find your way out of that maze. (2before.79-80)
Green is careful not to let one tone tug control away from the other. He balances irreverence with reflection throughout—just consider the Alaska Young Memorial Prank alongside Miles's final for the Old Man, or the complexity of how Miles feels about Alaska balanced with the awkwardness he and Lara have. Finding this equilibrium isn't easy, but Green manages.